1. Field of the Invention
This invention relates to a liquid crystal display device to prevent a liquid crystal module from being driven abnormally.
2. Discussion of the Related Art
A liquid crystal display device displays a picture corresponding to a video signal by controlling a light transmittance of a liquid crystal layer using an electric field applied to the liquid crystal layer. The liquid crystal display device is a kind of a flat panel display device having light, thin, and lower power consumption characteristic and so on, and has been used in portable computers such as a notebook computer, office automation products, audio products and video products. In particular, an active matrix type liquid crystal display device in which a switching element is formed in each of liquid crystal elements is very excellent in implementation of moving pictures because the switching element can be actively controlled.
A thin film transistor (TFT) is usually used as the switching element of the active matrix type liquid crystal display device. FIG. 1 shows an equivalent circuit of a pixel of the active matrix type liquid crystal display device in which the TFT is used as the switching element.
Referring to FIG. 1, the active matrix type liquid crystal display device converts a digital video data into an analog data voltage based on a gamma reference voltage, provide a data line DL with the analog data voltage, and provide a gate line GL with a scan pulse, thereby charging the data voltage into a capacitor. For this purpose, a gate electrode of the TFT is connected to the gate line GL, a source electrode of the TFT is connected to the data line DL and a drain electrode of the TFT is connected to a pixel electrode of a liquid crystal cell Clc and one electrode of a storage capacitor Cst. Also, a common electrode of the liquid crystal cell Clc is connected to a common voltage source so that common voltage Vcom is supplied to the liquid crystal cell Clc. Herein, the storage capacitor Cst maintains a voltage of the liquid crystal cell Clc constantly using a data voltage supplied from the data line DL when the TFT is turned on. When a scan pulse is supplied to the gate line GL to form a channel between the source and drain electrodes of the TFT, the data voltage supplied to the data line DL is applied to the pixel electrode of the liquid crystal cell Clc. At this time, liquid crystal molecules of the liquid crystal cell Clc are rearranged by the electric field formed between the pixel electrode and the common electrode of the liquid crystal cell Clc, thereby altering incident light.
For the above-mentioned operation, the liquid crystal display device comprises a liquid crystal module 20 for displaying pictures and a system 10 for generating driving signals necessary for driving the liquid crystal module 20 as shown in FIG. 2. FIG. 2 shows a block diagram of a related art liquid crystal display device.
The system 10 generates a data signal DATA, a module operation power VCC, a backlight operation power Vinv and so on, and supplies them to the liquid crystal module 20.
The liquid crystal module 20 comprises a timing controller 21, a DC-DC converter 22, a panel driving circuit 23, a liquid crystal display panel 24, a backlight driving circuit 25 and a backlight unit 26. The timing controller 21 is driven by the module operation power VCC. The timing controller 21 also rearranges the data signal DATA from the system 10, supplies it to the panel driving circuit 23 and controls an operation timing of the panel driving circuit 23 using a plurality of control signals. The DC-DC converter 22 is operated by the module operation power VCC supplied from the system 10 and generates a plurality of driving voltages necessary for driving the panel driving circuit 23. The panel driving circuit 23 drives data lines and gate lines formed in the liquid crystal display panel 24 according to the control signals from the timing controller 21 and the driving voltages from the DC-DC converter 22. The backlight driving circuit 25 is driven by the backlight operation power Vinv from the system 10 and generates a backlight driving voltage necessary for driving the backlight unit 26. The backlight unit 26 is driven by the backlight driving voltage and irradiates light on the liquid crystal display panel 24.
The related art liquid crystal display device is sometimes driven in a pause state in place of being continuously driven. For example, in case that the liquid crystal display device is applied to a navigation device, it is necessary to be operated only at the time when an user wants a navigation service. That is, in case that the user does not want a navigation service, it is desirable for the operation of the liquid crystal module 20 to be paused in order to save the consumption power. In general, in the pause mode, the module operation power VCC and the backlight operation power Vinv applied to the liquid crystal module 20 are cut off, but the data signal is continuously supplied to the liquid crystal module 20 in order to reduce a loading time of the system 10 when the navigation service restarts.
However, although the module operation power VCC and the backlight operation power Vinv applied to the liquid crystal module 20 are cut off in the pause mode, there is an abnormal phenomenon in an operation of the liquid crystal display panel 24 because the DC-DC converter 22 and the panel driving circuit 23 are driven by the data signal DATA induced along an operation power supplying line 12. Referring to FIG. 3, diodes for electrostatic discharge (ESD) are mounted in the timing controller 21 to protect circuit elements from internal static electricity. The diodes consists of a first diode D1 connected between a data bus line 11 to which the data signal DATA is supplied and a ground voltage supplying line 13, and a second diode D2 connected between the operation power supplying line 12 and the data bus line 11. The second diode D2 functions as a current path between the data bus line 11 and the operation power supplying line 12. Accordingly, even if the module operation power VCC is cut off, the data signal DATA is induced in the operation power supplying line 12. Herein, the data signal DATA has a voltage level (e.g., 2.6 volts) deducted by a threshold value of the second diode D2 from a transistor-transistor logic (TTL) level (e.g. 3.3 volts). As the operation power supplying line 12 is electrically connected to the DC-DC converter 22 as well as the timing controller 21, the DC-DC converter 22 and the timing controller 21 are driven by the induced data signal DATA in the state the module operation power VCC is cut off, thereby driving the liquid crystal display module 24. In case of continuing the situation, it is impossible to obtain an effectiveness of reducing consumption of power in the liquid crystal display module 20.